Microwave telecommunications circuits making use of components such as filters, amplifiers, mixers, . . ., introduce group delay distortion. The term "group delay" or GD is used to designate the value of the delay due to a component having a transfer function, said delay being proportional to the frequency derivative of the phase. For example, in a lowpass filter this delay is a function of frequency, giving rise to a bell-shaped curve with a maximum situated at the cut-off frequency of the filter.
In radio beam transmission, and in particular in digital transmissions, it is important to be able to correct the group delay in order to avoid bit errors. Correction is generally performed at intermediate frequency, or sometimes in base band if the distortion is symmetrical. When demodulation is performed directly at microwave frequency, without using an intermediate frequency, it is necessary to correct group delay directly at microwave frequency.
Such correction must be capable of correcting the phase of the microwave as a function of frequency without changing its amplitude since that would itself constitute a source of bit errors in a digital transmission. The commonly adopted solution consists in general in reflecting the microwave on a mismatched complex impedance, with the curve showing the phase of this complex impedance as a function of frequency being complementary to the curve of the group delay to be corrected. By adding the delay curve to the group delay curve, a delay curve is obtained which is uniform as a function of frequency.
A first known type of GD corrector uses a 90.degree. 3 dB coupler and two accurately identical complex impedances. This prior corrector is shown diagrammatically in accompanying FIG. 1. It uses a 90.degree. 3 dB coupler referenced 5. The input microwave signal E is applied to inlet port 1 of the coupler 5. It exits via ports 3 and 4 of the coupler with respective phase shifts of 0.degree. and of 90.degree. . These two waves are reflected from respective complex impedances 6 and 7 of value jX, and having a phase curve which is complementary to the phase curve to be corrected, with the waves finally recombining in-phase at outlet port 2 of the coupler 5 (microwave outlet S), and combining antiphase at the inlet port 1 of the coupler.
This first form of prior corrector suffers from the following drawbacks:
the two complex impedances 6 and 7 must have exactly identical complex impedances jX, otherwise the waves will no longer be exactly antiphase when they combine at the inlet port 1 and as a result the corrector will not act as an allpass transmission filter;
the coupler must provide perfectly symmetrical coupling and a completely accurate 90.degree. phase shift, otherwise the corrector will not behave as an allpass filter; and
the corrector is difficult to adjust: the two complex impedances 6 and 7 must be adjusted identically both in amplitude and in frequency, otherwise the corrector will not behave as an allpass filter.
Another known type of GD corrector uses a ferrite microwave circulator together with only one correction complex impedance. The diagram for this corrector is shown in accompanying FIG. 2.
The inlet microwave E is applied to port 10 of circulator 8. It exits via the second port 11, is reflected on complex impedance 9 of value jX, re-enters the circulator 8 via port 11, and leaves the circulator S via its third port 12.
This other prior corrector suffers from the following drawbacks:
it is difficult to adjust for low amplitude GD since the ferrite circulator has its own relatively large GD;
such a circulator is difficult to integrate in microstrip technology circuits, particularly when the microwave frequency is about 1 GHz; and
a ferrite circulator is relatively expensive, thereby increasing the overall cost of a GD corrector of this type.
The invention seeks to remedy these drawbacks.